SyntaxError: import declarations may only appear at top level of a module [duplicate]

I got this on Firefox (FF58). I fixed this with:

1. It is still experimental on Firefox (from v54):
   You have to set to true the variable dom.moduleScripts.enabled in about:config

Source: Import page on mozilla (See Browser compatibility)

2. Add type="module" to your script tag where you import the js file

<script type="module" src="https://stackoverflow.com/questions/42237388/appthatimports.js"></script>

3. Import files have to be prefixed (./, /, ../ or http:// before)

import * from "./mylib.js"

For more examples, this blog post is good.